1. Field of the Invention
This invention relates to a disk drive including a spindle motor that rotates a disk and a voice coil motor that acts as a driving source for a head actuator. More particularly, this invention relates to a method of and an apparatus for driving a spindle motor and a voice coil motor in a disk drive.
2. Description of the Related Art
One known typical disk drive that reads the information recorded on a disk serving as a recording medium by means of a head is a hard disk drive (or magnetic disk drive). The hard disk drive uses two types of motor: a spindle motor and a voice coil motor. The spindle motor is a brushless direct-current motor that rotates the disk. The voice coil motor is a driving source for a head actuator that moves the head along the radius of the disk.
A voltage E necessary to drive a brushless direct-current motor, such as a spindle motor, is expressed by the following equation:E=Ee+I×R  (1)
In equation (1), Ee is a voltage (hereinafter, referred to as a back EMF voltage) that corresponds to a back electromotive force (back EMF) generated in the motor coil as a result of the rotation of the motor. The back EMF voltage Ee is proportional to the torque constant and the rotational speed. I is the current flowing through the motor coil. R is the sum of the resistance of the motor coil and the resistance of the motor driver. The current I is proportional to the driving torque of the motor. Therefore, for example, when the motor load changes as a result of the change of the ambient temperature, the current I fluctuates accordingly. The torque constant and the coil resistance vary because of the characteristics of the motor. For this reason, the driving voltage of the motor is designed to be higher than E to allow a margin, taking those variations into account.
The margin, however, results in power loss in the motor driver that drives the motor. Thus, when the driving voltage of the motor is designed, allowing for a margin, this causes the problem of increasing the power consumption. This problem particularly becomes significant in fluid dynamics bearing spindle motors which are becoming increasingly popular nowadays as spindle motors used in hard disk motors. The reason is that, in a fluid dynamics bearing spindle motor, the viscosity of fluid (e.g., oil) increases with the ambient temperature and therefore the load on the motor fluctuates significantly. Obviously, a great change in the motor load results in a large fluctuation in the driving voltage E. Thus, it is necessary to allow a large margin for the motor driving voltage actually used, taking a fluctuation in the driving voltage E into account. Allowing a large margin for the motor driving voltage increases the power loss in the motor driver accordingly. A technique for reducing the power loss in a motor driver has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-208091. In the technique written in the publication (hereinafter, referred to as the prior art), the driving voltage (or supply voltage) is changed by a power supply unit capable of changing the voltage. The driving voltage is the voltage necessary for the motor driver to drive the motor. The voltage is varied according to the increase or decrease in the current flowing through the motor coil. By this variable control, the difference between the terminal voltage (or coil terminal voltage) of the motor and the driving voltage is minimized, which reduces the loss in the motor driver.
In a hard disk drive that rotates the spindle motor at high speed, the faster the motor rotates, the higher the driving voltage has to be. Moreover, to drive, for example, a fluid dynamics bearing spindle motor in a severe low-temperature environment, a large driving torque is needed. In this case, a high driving voltage is necessary to drive the spindle motor. To meet this requirement, the following can be considered: the voltage supplied from the host using the hard disk drive is stepped up by a voltage booster and the boosted voltage is used to drive the spindle motor. The method of boosting the voltage has the advantage of being capable of minimizing the loss in the motor drive. The reason is that it is possible to boost the voltage to a value at least necessary to drive the spindle motor at a steady rotational speed and the motor is driven by the boosted voltage. Another advantage of the method of boosting the voltage is that a seek operation of moving the head to the target track on the disk can be performed at high speed. The reason is that the higher the driving voltage, the faster the seek speed can be made by causing a large current to flow through the voice coil motor. Therefore, using a single voltage booster to drive both the spindle motor and the voice coil motor makes it possible to realize the following two properties: one is that the spindle motor can be rotated at high speed or the spindle motor can be driven in a low temperature environment, and the other is that a seek operation can be made faster by the voice coil motor.
However, when the prior art is applied to reduce the loss in the motor driver, a problem arises. Specifically, when the supply voltage is varied according to the voltage necessary to drive the spindle motor, the following problem arises: the driving voltage of the voice coil motor also varies with fluctuations in the spindle motor. Conversely, when the voltage booster steps up the supply voltage to a maximum so as to cause a large current to flow through the voice coil motor, the following problem arises: since the actually used driving voltage becomes larger than the voltage at least necessary to drive the spindle motor, the power loss in the motor driver becomes larger.